Combustion apparatus



April 12, 1955 A. P. JoHNsToNE 2,705,869

COMBUSTION APPARATUS Original Filed Feb. 7, 1949 Attorneys United States Patent YO CONIBUSTION APPARATUS Alexander Pearse Johnstone, Frimley, England, assignor to Power Jets (Research and Development) Limited, London, England, a British company Claims priority, application Great Britain August 17, 1948 8 Claims. (Cl. Gil-39.72)

This invention relates to combustion apparatus in which combustion is required to be supported by a current of air or other gas (hereinafter referred to as air) of flame extinguishing velocity. The term flame extinguishing velocity is used to indicate that the mean speed of the combustion-supporting air current in its general direction of flow past a combustion zone, calculated from the ratio air volume passing in unit time/cross sectional area of flow path, is substantially higher than the speed of flame propagation in the fuel/air mixture concerned. For hydrocarbon fuels burning in air the speed of flame propagation is considered as being of the order of one foot per second at atmospheric temperature; the invention on the other hand, is especially applicable to combustion apparatus in which the speed of the air current in its general direction of flow past a combustion zone, calculated on the basis indicated, might be of an order as low as 10 or as high as 500 feet per second or even more, depending upon the design.

An object of the invention is to avoid difficulties which arise when fuel has to be burnt in a flow of air through a duct whose cross-sectional area is large in the sense that having regard to the speed of flame propagation, the velocity of the air flow, and the length of passage available within which to complete combustion, it is not possible to achieve a spread of flame across the whole cross-section of the duct using only a single source of ignition. In one aspect therefore this invention is an improvement on that described in copending United States patent application, filed February 7, 1949, Serial No. 75,042, for Combustion Apparatus, now abandoned, in the names of Probert, Johnstone and Roberson. This application is a division of said application Serial No. 75,042. In said application there is provided in combination with a gas turbine jet propulsion unit having a jet pipe and a conical fairing arranged within said pipe to form with said pipe an annular diffuser, combustion apparatus comprising a further diffuser downstream of said firstmentioned diffuser, a plurality of fuel injecting means distributed over a transverse section of said further diffuser and injecting fuel without ignition into said further diffuser and a plurality of tiame stabilizing means distributed over a transverse section of said further diffuser downstream of the flame stabilizing means, each of said stabilizing means forming a combustion stabilizing zone in which a stable flame can be maintained notwithstanding a fiame extinguishing mean overall velocity of the gas ow past said zone. The present invention however is not limited to the jet pipe of a gas turbine jet propulsion unit in its application, but may be used in any large duct conveying a high velocity air flow.

If a system having a number of stabilizing zones and fuel injecting means distributed over the crosssection of a duct is applied to re-heating the exhaust gases of a simple jet propulsion unit, a certain minimum fuel fiow is required depending on the rate of air mass flow through the duct. If this flow is high (as is generally the case) the minimum fuel flow will also be high and since the thrust increase depends on the fuel flow, the minimum thrust increase will also be high. On the other hand if for operational requirements a relatively low thrust increase is required, fuel flow is correspondingly decreased and it becomes desirable to install a concentrated air-fuel mixture at one or more local points in order to permit ignition to take place. This may be accomplished by limiting the spread of the fuel across the duct.

Accordingly the invention provides a combustion apparatus in which combustion is required to be supported by a gaseous stream of flame extinguishing velocity comprising a duct conveying said stream, a plurality of flame stabilizing means distributed over a transverse section of the duct, a plurality of fuel injecting means distributed over a transverse section of the duct upstream of the flame stabilizing means and injecting fuel without ignition into the duct, and means for controlling the lateral spread of the fuel between said fuel injecting means and flame stabilizing means.

The means for controlling the spread of the fuel preferably comprise an open ended tubular member surrounding each fuel injecting means and having its axis parallel to that of the duct.

In a gas turbine jet pipe it may be necessary to decrease the speed of the exhaust gases to obtain stable combustion, and consequently a diffuser section may be provided in the jet pipe in which the fuel injecting means and ame stabilizing means are located.

The invention is illustrated by way of example in the accompanying drawing in which:

Figure 1 is a diagrammatic axial section of one form of the invention as applied to the burning of fuel in the exhaust pipe of a gas turbine jet propulsion unit.

Figure 2 is an enlarged detail of the form shown in Figure 1.

Figure 3 is a diagrammatic axial section of a modification of the form shown in Figure 1.

Figures 4 and 5 are enlarged details showing further modifications.

In Figure l a turbine 1 has on its downstream side a fairing or bullet 2 forming with the exhaust cone 3 an annular diffusion passage. At the downstream end of this passage the jet pipe has a divergent section 4 which forms a further diffusion passage, and downstream of this there is a section 5 of constant crosssectiou terminating in a convergent portion 6 leading to a jet orifice 7. Fuel is injected upstream by a plurality of jets 9 located in an upstream portion of the diffusion passage 4, each of which jets is surrounded by an open ended tubular member 12 supported by a streamlined fairing 18 and having its axis parallel to that of the duct. At the junction of the diffusion passage 4 and the section 5, there is provided a plurality of flame stabilizing baflies 10, each baffle being designed to produce in its wake a reverse flow system whereby stable combustion is maintained in spite of the high velocity of the gas flow. To effect ignition, each baffle is provided with spark gap, to which current is taken by lead 20. The baies are conical in shape, having their larger ends facing downstream, and there are an equal number of baffles and fuel jets.

Fuel issuing upstream from the jets 9 is reversed by the stream of hot turbine exhaust gases and flows downstream past the baffles 10. The distance between each jet 9 and its corresponding baffle 10 is sufficient to allow time for vaporisation of the fuel, and the length of the tube 12 downstream of each jet 9 limits the lateral spread of fuel and hence a desired degree of concentration of the fuel-air mixture at the baliie 10 may be achieved.

The arrangement also prevents fuel from reaching the duct wall and causing local overheating.

It will be seen that the tubular members 12 constitute walls which split the duct into a plurality of longitudinally extending passages in parallel with one another. The tubular members themselves define passages which contain fuel injectors 9, while they define with .the jet pipe 4 a further passage not containing an injector.

Normally it will .not be desired to permit fuel to leave the upstream end of tube 12 and that part of the tube which is upstream of jet 9 is made suciently long to prevent escape of fuel at this end. However, at very' high fuel pressures the upstream path of the fuel inside tube 12 before reversal is increased and it may be difficult to prevent such escape without having too great a tube length. Moreover in such circumstances it may even be advantageous tol allow a controlled amount of fuel to pass through the upstream end since such fuel will after reversal by the air current, travel downstream over the outside of tube 12 to form a fuel layer at the downstream end of the tube and in some cases will thus facilitate combustion.

In order to effect such controlled escape of fuel from the upstream end of tube 12, this end may be flared outwardly as shown at 13 in Figure 4. Alternatively or in addition each tube 12 may be provided with an outer sheath 14 as shown in Figure 5 or the whole system of fuel jets 9, tubes 12 and baflies 10 may be arranged within a larger sheath 15 as shown in Figure 3.

To exert a further control on the escape of fuel from the tube 12, the modification described in copending United States patent application Serial No. 220,605, filed April 12, 1951, in the name of Forman may be used.

I claim:

1. Combustion apparatus for burning fuel in a fast moving gas stream comprising a duct carrying the stream, mixing means that includes walls extending along the duct splitting it into a plurality of longitudinally extending passages in parallel with one another and a plurality of fuel injectors each located in one of said passages, the number of injectors being less than the number of passages, a plurality of flame stabilizers distributed over a transverse section of the duct spaced downstream from the downstream ends of said walls, land an igniter in the neighborhood of said ame stabiizers.

2. Combustion apparatus for burning fuel in a fast moving gas stream comprising a duct carrying the stream, mixing means including a plurality of fuel injectors symmetrically disposed over one transverse section of the duct and a plurality of open ended tubular members extending along the stream and each enclosing one of said injectors, a plurality of flame stabilizers symmetrically disposed over a transverse section of the duct spaced downstream from the downstream ends of said tubular members, and an igniter in the neighborhood of said ame stabilizers.

3. Combustion apparatus according to claim 2 wherein the upstream ends of said tubular members are flared outwardly.

4. Combustion apparatus according to claim 2 further comprising a plurality of open ended tubular sheath members, each enclosing one of said tubular members which enclose the fuel injector.

5. Combustion apparatus according to claim 2 wherein the fuel is injected upstream relative to the general direction of gas ow` in vthe duct.

6. Combustion apparatus for burning fuel in a fast moving gas stream comprising a duct carrying the stream, a plurality of fuel injectors symmetrically disposed over one transverse section of the duct, a plurality of open ended tubular members extending along the stream, and each enclosing one of said injectors, a plurality of ame stabilizers symmetrically disposed over a transverse section of the duct spaced downstream from said tubular members and each being aligned with the axis of one of said members, and an igniter in the neighborhood of said flame stabilizers.

7. Combustion apparatus for burning fuel in a fastmoving gas stream comprising a duct -carrying the stream, mixing means that includes walls extending along the duct splitting it into a plurality of longitudinally extending passages in parallel with one another and a plurality of fuel injectors each located in one of said passages, the number of injectors being less than the number of passages, a plurality of ame stabilizers disposed over a transverse section of the duct spaced downstream from the downstream ends of said walls and each being aligned with one of the passages containing an injector, and an igniter in the neighborhood of said flame stabilizers.

8. Combustion apparatus for burning fuel in a fastmoving gas stream comprising a duct carrying the stream, mixing means that includes a plurality of fuel injectors symmetrically disposed over a transverse section of the duct and a plurality of tubular members extending along the stream and each enclosing one of said injectors, each said tubular member being open at its upstream end to admit part of said stream to its interior to mix with the fuel from the injector to form a gas-fuel mixture and at its downstream end to discharge a stream of said gas-fuel mixture, a plurality of ame stabilizers symmetrically disposed over a transverse section of the duct downstream of the downstream ends of said tubular members and each being aligned with one of said streams of gas-fuel mixture, and an igniter in the region of said ame stabilizers.

References Cited in the file of this patent UNITED STATES PATENTS 2,385,833 Nahigyan Oct. 2, 1945 2,506,611 Neal et al. May 9, 1950 2,508,420 Redding May 23, 1950 2,526,122 Darlington Oct. 17, 1950 2,557,020 Viles June 12, 1951 2,566,373 Redding Sept. 4, 1951 2,583,651 Horning Jan. 29, 1952 FOREIGN PATENTS 920,910 France Jan. 8, 1947 

